Argonaute-mediated translational repression (and activation).

MicroRNAs (miRNAs) downregulate the expression of their target genes by inducing translational repression and/or mRNA decay. Under specific conditions, miRNAs can even activate translation of their target mRNAs. These processes occur via miRNA-protein complexes, or RNA-induced silencing complexes (RISCs), which contain Argonaute (Ago) subfamily protein as a core component. However, detailed mechanisms of miRNA-mediated translational regulation remain unclear. We recently reported that, in Drosophila, both of the two Ago proteins, Ago1 and Ago2, can repress translation of the target mRNAs, but by remarkably different mechanisms. Furthermore, we here show that Ago2, but not Ago1, can activate translation of the target mRNAs when they lack the poly(A) tail, suggesting that the length of poly(A) tail is an important determinant for the consequences of Ago2 function. This review focuses on how miRNAs regulate translation in light of these new findings.